Redstone Control
Redstone can be used to activate factories, and to tell when they're running.
Normally, factories are insensitive to redstone and can only be activated by a player. You can make a factory respond to redstone by sticking a lever on the furnace block (any face but the bottom). This lever can only be placed by someone with access to the furnace to prevent unauthorised activation.
Once the right materials are in the factory for the current recipe, it can be started by activating any redstone wire, lever, or button adjacent to the furnace. Due to some fiddly redstone mechanics, repeaters, comparators and direct redstone torches don't work at present.
When the factory starts, the lever on the furnace block will be switched to the on position. This will charge the furnace, lighting up connected redstone. When the factory stops, it will switch the lever to the off position. This will keep connected redstone lit while running.
When the factory finishes, the lever will turn off again. This will let the redstone turn off.
Note that the lever will turn off when the factory powers off even if it failed. This could involve manual deactivation, not enough fuel, or modification of the chest contents while operating.